The basic concept of electrostatic spinning (or electrospinning) a polymer to form extremely small diameter fibers was first patented by Anton Formhals (U.S. Pat. No. 1,975,504). Electrostatically spun fibers and nonwoven webs formed therefrom have traditionally found use in filtration applications, but have begun to gain attention in other industries, including in nonwoven textile applications as barrier fabrics, wipes, medical and pharmaceutical uses, and the like.
Electrospining is a process by which electrostatic polymer fibers with micron to nanometer size diameters can be deposited on a substrate such as a flat plate. By way example, Westbroek, et el (US20100112020) illustrate deposition of electrospun fibers on a flat plate as shown in FIG. 1. Such fibers have a high surface area to volume ratio, which can improve the structural and functional properties of a fiber structure collected on a substrate. Typically, a jet of polymer solution is driven from a highly positive charged metallic needle (i.e. an emitter) to the substrate which is typically grounded. Sessile and pendant droplets of polymer solutions may then acquire stable shapes when they are electrically charged by applying an electrical potential difference between the droplet and the flat plate. These stable shapes result only from equilibrium of the electric forces and surface tension in the cases of inviscid, Newtonian, and viscoelastic liquids. In liquids with a nonrelaxing elastic force, that force also affects the shapes. When a critical potential has been reached and any further increase will destroy the equilibrium, the liquid body acquires a conical shape referred to as the Taylor cone.
Synthetic polymers including collagen, gelatin, chitosan, poly (lactic acid) (PLA), poly(glycolic acid) (PGA), and poly(lactide-co-glycolide) (PLGA) have been used for electrospinning. In addition to the chemical structure of the polymer, many parameters such as solution properties (e.g., viscosity, conductivity, surface tension, polymer molecular weight, dipole moment, and dielectric constant), process variables (e.g., flow rate, electric field strength, distance between a fiber emitter [e.g., needle] and collector [e.g., flat plate, drum], emitter tip design, and collector geometry), and ambient conditions (e.g., temperature, humidity, and air velocity) can be manipulated to produce fibers with desired composition, shape, size, and thickness. Polymer solution viscosity and collector geometry are important factors determining the size and morphology of electrospun fibers. Below a critical solution viscosity, the accelerating jet from the tip of the capillary breaks into droplets as a result of surface tension. Above a critical viscosity, the repulsive force resulting from the induced charge distribution on the droplet overcomes the surface tension, the accelerating jet does not break up, and results in collection of fibers on the grounded target. A variety of target types have been used, with flat plate and drum targets being common. By way of example, Korean Patent KR101689740B1 illustrates use of a drum target in electrospinning as shown in FIG. 2. Although the fiber shown in FIG. 2 appears as a single thread, the jet of fiber divides into many branches on its surface after the jet leaves the tip of the needle (Yarin, K Yarin, A. L., W. Kataphinan and D. H. Reneker (2005). “Branching in electrospinning of nanofibers.” Journal of Applied Physics 98(6):—ataphinan et al. 2005). If not controlled, the branches of the fibers create a non-uniform deposition on the target collector. One objective of the present invention is to enable a more controlled deposition of fibers to achieve a more uniform and cross-aligned distribution of the fiber on a collector.
Many engineering applications require uniform distribution of the fiber on the substrate. For example, one of the most important cell morphologies associated with tissue engineering is elongated unidirectional cell alignment. Many tissues such as nerve, skeletal and cardiac muscle, tendon, ligament, and blood vessels contain cells oriented in a highly aligned arrangement, thus it is desirable that scaffolds designed for these tissue types are able to induce aligned cell arrangements. It is well documented that cells adopt a linear orientation on aligned substrates such as grooves and fibers. Aligned nanofiber arrays can be fabricated using the electrospinning method [Li D, Xia Y. Electrospinning of nanofibers: reinventing the wheel? Adv Mater. 2004; 16:1151-1170] and many studies have shown that cells align with the direction of the fibers in these scaffolds. It is known that electrospun fibers can be aligned by attracting the fibers to a pair of electrically grounded, opposing and rotating disks or a pair of electrically grounded, parallel wires. It is known that cross-alignment of fibers can be achieved by first attracting fibers between parallel collectors such as rotating disks or parallel wires, then harvesting those fibers on a substrate, rotating the substrate 90 degrees and then harvesting more fibers to produce cross-aligned fiber layers. By way of example, Khandaker, et al. in U.S. Pat. No. 9,359,694 illustrate use of opposing disks in fiber collection as shown in FIG. 3A. Further, Khandaker, et al. in U.S. Pat. No. 9,809,906 illustrate use of parallel wires in fiber collection as shown in FIG. 3B. Cross alignment of fibers in layers can also be achieved as reported by Zhang, et al where biaxial orientation mats were electrospun using a collector consisting of two rotating disks with conductive edge to collect fibers in one orientation, and an auxiliary electrode to induce an electrostatic field to force the fibers to align in another orientation. (Jianfeng Zhang, Dongzhi Yang, Ziping Zhang, and Jun Nie (2008). “Preparation of biaxial orientation mats from single fibers.” Polym. Adv. Technol 2010, 21 606-608.) The biaxial orientation structure was formed with variation of rotation speed for each layer, without revolving the fiber mat during the electrospinning process. However, the degree of biaxial orientation was found to be strongly dependent on the rotation speed of the disks. A significant deficiency in the method was reported to be the destruction of a first fiber layer while forming a second cross-aligned fiber layer. This appears to be a limiting factor in fabricating larger size mats because the fibers in the first layer cannot withstand the forces imparted by higher rotation speeds needed to apply the second layer. Parallel collector plates have also been used, and may be combined with manual or robotic harvesting of fibers. By way of example, Korean Patent KR101224544B1 illustrates the use of parallel plates in fiber collection as shown in FIG. 4. Opposing disks, and both parallel wires and plates may be used to achieve fiber alignment and cross-alignment, but these known methods all suffer significant challenges in scalability for commercial applications, particularly as the physical dimensions of width and length of the desired mat are increased.
In addition to the influence on fiber arrangement, cell alignment can have positive effects on cell growth within tissue engineering scaffolds. Myotubes formed on aligned nanofiber scaffolds were more than twice the length of myotubes grown on randomly oriented fibers (p<0.05) and neurites extending from DRG explants on highly aligned scaffolds were 16 and 20% longer than those grown on intermediate and randomly aligned scaffolds respectively [Choi J S, Lee S J, Christ G J, Atala A, Yoo J J. The influence of electrospun aligned poly(epsilon-caprolactone)/collagen nanofiber meshes on the formation of self-aligned skeletal muscle myotubes. Biomaterials. 2008 July; 29(19):2899-906].
Growth of electrical bending instability (also known as whipping instability) and further elongation of the jet may be accompanied with the jet branching and/or splitting. Branching of the jet of polymer during the electrospin process has been observed for many polymers, for example, polycaprolactone (PCL)(Yarin, Kataphinan et al. 2005), polyethylence oxide (Reneker, D. H., A. L. Yarin, H. Fong and S. Koombhongse (2000) “Bending instability of electrically charged liquid jets of polymer solutions in electrospinning.” Journal of Applied physics 87(9): 4531-4547). Such branching produces non-uniform deposition of fiber on a collector during the electrospin process.
Chronic wound care consumes a massive share of total healthcare spending globally. Care for chronic wounds has been reported to cost 2% to 3% of the healthcare budgets in developed countries (R. Frykberg, J. Banks (2015) “Challenges in the Treatment of Chronic Wounds” Advances in Wound Care, Vol. 4, Number 9, 560-582). In the United States, chronic wounds impact nearly 15% of Medicare beneficiaries at an estimated annual cost of $28 billion. In Canada, the estimated cost to the health system is $3.9 billion. Despite significant progress over the past decade in dealing with chronic (non-healing) wounds, the problem remains a significant challenge for healthcare providers and continues to worsen each year given the demographics of an aging population. Persistent chronic pain associated with chronic wounds is caused by tissue or nerve damage and is influenced by dressing changes and chronic inflammation at the wound site. Chronic wounds take a long time to heal and patients can suffer from chronic wounds for many years. Wound dressings are often extremely painful to remove, particularly for severe burn wounds. The removal of these dressings can peel away the fresh and fragile skin that is making contact with the dressing, causing extreme pain and prolonged recovery time. There is also a greater risk for infection and the onset of sepsis, which is can be fatal.
Research at the University of Manitoba has demonstrated positive effects of antimicrobial nanofiber membranes in treating the conditions of infection in chronic wounds (Zahra Abdali, Sarvesh Logsetty, and Song Liu, Bacteria-Responsive Single and Core-Shell Nanofibrous Membranes Based on Polycaprolactone/Poly(ethylene succinate) for On-Demand Release of Biocides, ACS Omega 2019 4 (2), 4063-4070). A PHA based core-shell structural nanofibrous mat incorporating a broad-spectrum potent biocide in the core of the nanofibers was fabricated by coaxial electrospinning. The nanofiborous mats produced comprised randomly oriented PHA based core-shell nanofibers. The random structure of the fibers limited surface contact with a wound and any resulting triggered release of biocides present in the outer layers of the mat. Further, the random orientation of the nanofibers presented less than optimal porosity for cell migration and exudate flow from a wound. FIG. 5 illustrates the electrospinning method used to produce core-shell (PHA)-based nanofibers mats for wound dressing applications as reported by Abdali, et. el. at University of Manitoba.
An electrospinning apparatus developed by the National Aeronautics and Space Administration (NASA) is directed to producing larger size fiber mats comprising aligned fibers. NASA's Langley Research Center created a modified electrospinning apparatus (shown in FIG. 6) for spinning highly aligned polymer fibers as disclosed in U.S. Pat. No. 7,993,567. NASA developed an apparatus that uses an auxiliary counter electrode to align fibers for control of the fiber distribution during the electrospinning process. The electrostatic force imposed by the auxiliary electrode creates a converged electric field, which affords control over the distribution of the fibers on the rotating collector surface. A polymer solution is expelled through the tip of the spinneret (i.e. emitter) at a set flow rate as a positive charge is applied. An auxiliary electrode, which is negatively charged, is positioned opposite the charged spinneret. The disparity in charges creates an electric field that effectively controls the behavior of the polymer jet as the jet is expelled from the spinneret. The electric field controls the distribution of the fibers and mats formed from the polymer solution as fibers land on a rotating collection mandrel (i.e. drum collector). The disclosure recites “Pseudo-woven mats were generated by electrospinning multiple layers in a 0°/90° lay-up. This was achieved by electrospinning the first layer onto a Kapton® film attached to the collector, manually removing the polymer film from the collector, rotating it 90°, reattaching it to the collector and electrospinning the second layer on top of the first, resulting in the second layer lying 90° relative to the first layer. Fibers were collected for one minute in each direction. A high degree of alignment was observed in this configuration. In order to assess the quality of a thicker pseudo-woven mat, the lay-up procedure was repeated 15 times in each direction)(0°/90° for a period of 30-60 seconds for each orientation, generating a total of 30 layers.” The required and repeated step of “removing the polymer film, rotating it 90°, reattaching it to the collector and electrospinning the second layer on top of the first” is a major deficiency in the method and apparatus taught in the NASA'567 patent when considered from the perspective of cost-effective commercial production of cross-aligned nanofiber membranes. While the drum supports attached fibers and prevents layer destruction during rotation unlike the method reported by Zhang, et al., repeated manual removal of the Kapton® film reportedly results in some misalignment of the collected fibers, which distorts the cross-alignment of fibers in the resulting fiber mat. Further, the labor cost and production time associated with repeated manual removal of the Kapton® film and reattachment on the collector is cost prohibitive in commercial applications of electrospinning. NASA has not licensed U.S. Pat. No. 7,993,567.
A method and apparatus to fabricate larger-size, well-structured membranes comprising cross-aligned electrospun fiber from many fiber branches, without fiber layer destruction and manual processes, has not been solved. Larger dimension membranes are needed for example in fabricating a range of fibrous drug delivery dressings including wound care dressings, as well as at least tissue engineering scaffolds, medical grade filters, and protective fabrics. A scalable method is needed by which uniformly distributed fiber can be deposited on a collector during electrospinning processes, achieving cross-aligned fiber deposition and larger-size fiber membranes absent manual intervention.